System, sensor combination and method for regulating, detecting as well as deciding current fuel-air ratios in combustion engines

ABSTRACT

A system for regulating the fuel-air mixture in a multi-cylinder internal combustion engine. The system utilizes binary sensors to detect relative deviations from stoichiometric combustion, including individual combustion events, and allows for regulation to achieve optimal and similar combustion to take place in all the cylinders.

The present invention concerns a system for regulating the fuel-airmixture in internal combustion engines, a sensor combination for asimilar system, and an arrangement for determining the fuel-air mixturein an internal combustion engine.

TECHNOLOGICAL STANDPOINT

With the aim of regulating the combustion in an internal combustionengine, so that an optimal stoichiometric combustion takes place for thecatalytic converter, sensors in the exhaust system are used, whichdetect the proportion of residual oxygen in the exhaust gases.Stoichiometric combustion is desirable in order that the catalyticconverter shall operate most efficiently and minimise the emission ofNO_(x), HC and CO. The sensors used for this purpose are principallysensitive to the transport of oxygen ions, and are generally calledlambda sensors. A characteristic of these sensors is that they arerelatively slow to act, and in reality provide an averaged signal thatspans several sequential combustion events. A normal step response fromsuch a sensor is that there is a delay in the order of 20 to 30combustion events before the sensor achieves a new stable output signallevel after a change in the actual air-fuel mixture. One disadvantagewith this type of sensor is that if it is installed in the exhaustsystem downstream (with respect to the direction of gas flow) of theexhaust manifold in a multi-cylinder engine, in a position where theexhaust gases from all the cylinders have combined, this can oftenresult in regulation so that individual cylinders run rich while theothers run lean, although the combined gas flow indicates stoichiometriccombustion has been achieved. The alternative is to arrange a separatesensor in the exhaust gas flow from each individual cylinder, but thiswould be very expensive. A conventional binary lambda sonde costs at theconsumer level about SEK 1200-1400 (□135-158), and linear lambda sondescost between 10 and 20 times as much as binary sensors.

By using sensors of the type shown in SE.A.9403218-2(=PCT/SE95/01084)any change in the fuel-air mixture can be detected much more quickly.This sensor is also of a binary type, where the sensor output signalquickly changes from one level to another depending on whether theproportion of hydrogen (H₂) in the exhaust gases exceeds or is less thana predetermined value.

OBJECT OF THE INVENTION

The object of the present invention is with only one binary sensor to beable to quickly detect relative deviations from stoichiometriccombustion, even for individual combustion events in a multi-cylinderinternal combustion engine. From this basis it will easily be possibleto regulate all the cylinders equally, so that optimal and similarcombustion can take place in all the cylinders. Uneven combustion in aset of cylinders can result in individual cylinders running rich andthereby building up soot deposits. This soot can give rise to so-calledhot spots, inducing knocking. In those cylinders which are running lean,the lean combustion itself can increase the risk of knocking. For everytype of anti-knock measure the engine deviates from optimal regulationand its fuel consumption increases.

Another reason is to limit emissions, which will be the result if allcylinders can be regulated for stoichiometric combustion. Even smalldeviations from stoichiometric combustion, for example with excess aircontent variations in the region of Δλ≈0.001-0.002, will reducecatalytic converter efficiency from 98% to 80-85%.

A further reason is closer regulation of the fuel supply tomulti-cylinder internal combustion engines using fuel injectors,permitting lower tolerance claims in the manufacture of the fuelinjector components. The need is reduced for a continuous tightening ofmanufacturing tolerances for fuel injectors, or the alternative ofmatching individual fuel injectors with similar dynamic responses, withthe aim of meeting ever more stringent emission claims.

Yet another purpose is that with a special sensor combination it will bepossible to detect relative deviations in both the rich and leandirections away from stoichiometric combustion.

BRIEF DESCRIPTION OF THE INVENTION

By means of the present invention the fuel supply to each cylinder canbe regulated in an optimal manner such that stoichiometric combustiontakes place in each cylinder.

By means of the sensor combination of the present invention, relativedeviations relative to stoichiometric combustion can be detected, inboth rich and lean burn directions, using only a sensor elementproviding a binary type of output signal.

By means of the general process of the invention detection of therelative deviation from stoichiometric combustion in every cylinder isassured, based upon a sensor of binary type.

Other particularly remarkable characteristics and advantages derivingfrom the present invention are apparent in the other patent claimcharacteristic parts and in the subsequent description of an applicationexample. The description of the application example utilises referencesto the illustrations defined in the following list of drawings.

LIST OF DRAWINGS

FIG. 1 shows diagrammatically an internal combustion engine with asystem for regulating the fuel-air mixture.

FIG. 2 shows the reaction principle in a sensor that is used inaccordance with the present invention.

FIG. 3 shows the design of a sensor which, depending on the actual levelof hydrogen present, provides a distinct changeover point in its outputsignal.

FIG. 4 shows the output signal from a sensor of the type shown in FIG. 3when in use as an exhaust gas sensor (sensor 10) in a system equivalentto that shown in FIG. 1.

FIGS. 5a and 5 b respectively show the excess air factors from the fourcylinders from the first curve from the top and second curve from thetop respectively in FIG. 4.

DESCRIPTION OF AN APPLICATION EXAMPLE

FIG. 1 shows diagrammatically an internal combustion engine 1 equippedwith a regulatory system for its fuel supply. In the conventional wayfuel is delivered to cylinders 2 a, 2 b, 2 c and 2 d with the aid offuel injectors 3 a, 3 b, 3 c and 3 d respectively, arranged in the inletmanifold 6, and directed toward the respective inlet valves for thecylinders. Injectors 3 a-3 d are located in a fuel distribution railpipe 5 which is supplied with fuel from a fuel tank 4 by means of a pump4. The contents of the fuel rail pipe 5 are under continuous pressure ata principally constant pressure level and the amount of fuel that issprayed into the combustion chamber through the inlet valve isdetermined by the time period of an electrical control pulse transmittedfrom and controlled by an engine control unit, ECM. The Figure shows asystem in which the pump can be controlled by pressure, butalternatively a system with excess fuel returning to the tank 4 via apressure-reducing valve can be used. The Figure shows a fuel system ofso-called low pressure type, whereby an indirect supply of fuel to thecylinders takes place through the fuel injectors pointing towards theinlet valves. Engines with fuel injected directly into the cylinders mayalso be used.

The engine control unit ECM adapts the actual length of time of thecontrolling pulse to the respective fuel injectors 3 a-3 d in responseto a number of parameters. The actual engine rotation speed andcrankshaft position are determined by a pulse sender 9, which in aconventional manner detects the presence of the gear teeth on theperiphery of the flywheel 8. Sensors 14 and 15 detect the acceleratorpedal position and engine coolant temperature respectively. The actualmass of the air entering the cylinders is detected by an air mass sensor12, and this is used to determine the load on the engine. Depending onthe values at any instant of these specified detected engine parametersthe engine control unit then ensures that a suitable quantity of fuel isdelivered, as determined by an empirical engine load, engine speed andcoolant temperature matrix, along with the influence of the driver onthe accelerator pedal position 14.

With the aim of reducing emissions from the combustion process, aso-called three-way catalytic converter is installed in the conventionalmanner in the exhaust piping 7 g. The catalytic converter can reduce thelevels of NO_(x), and CO, while HC is oxidised with very high efficiencyof approximately 98% in the presence of a stoichiometric combustionrelationship of air to fuel. The proportion of residual oxygen in theexhaust gases is a function of the air-fuel mixture ratio, so that thelevel of oxygen in the exhaust gases can be used to determine the excessair factor (λ). Normally an oxygen sensor of binary type, called alambda sonde, is used, which provides an output signal with a distinctswitching point when the excess air factor λ falls below 1.0. This typeof binary sensor usually presents a principally low voltage output whilethe excess air factor is greater than 1.0, and delivers a higher outputvoltage if the excess air factor falls below 1.0. This is used tocorrect the value of fuel to be supplied primarily determined by thematrix, whereupon the engine control unit with as small changes in fuelsupply as possible tries to keep the output signal from the lambda sondecontinuously switching between low and high signal outputs. Usually,regulation using this type of switching in normal operation means theoutput signal changes at a rate in the order of once per second. Adisadvantage of this type of sensor is that it is relatively slow, andthere may be a delay of ten or more combustion events before the signalchanges from indicating too much to too little air, which makes itunsuitable for detecting the combustion products from an individualcylinder, if it is installed as shown in FIG. 1 in the exhaust piping 7g.

FIG. 2 shows schematically the structure of a sensor and its gasdetection principle together with the chemical reactions within such asensor that is used in accordance with the present invention. The sensoris sensitive to hydrogen (H₂) and the principle of this type ofsemiconductor sensitivity has been described in “A Hydrogen SensitiveMOS-Transistor, J. Appl. Phys. 46 (1975) 3876-3881. K. I. Lundström, M.S. Shivaraman & C. Svensson”. The principle is that hydrogen H₂ diffusesdown through the metallic film and forms an electrically polarised layeron the insulated stratum (SiO₂). The polarised layer causes a voltagedrop ΔV. For the real high temperature application, a silicon carbide(SiC) substrate is used. During the manufacture of the sensor, the SiCsubstrate is cleaned and oxidised so that a film of SiO₂ is formed.Thereupon a resistive contact consisting of a 200 nm layer of TaSi_(x)and a 400 nm layer of Pt is deposited.

In order to obtain a functional sensor in accordance with FIG. 3 a pitis etched in from above, with a diameter of approximately 0.7 mm. FIG. 3shows both a side elevation and a plan view of the physical sensor. Thecontact area consists of a 200 nm layer of TaSi_(x) and a 400 nm layerof Pt deposited by means of DC-magnetron sputtering at a temperature of350° C. Thereafter, using the same technique, a control electrode isdeposited, consisting of a 10 nm layer of TsSiX and 100 nm Pt, whichpartly overlaps the contact surfaces. Finally platinum (Pt) ribbons arewelded to the contact surfaces. The sensor can then be mounted usingceramic glue on a conventional ceramic support, preferably a ceramicsupport with temperature regulation, equivalent to the support used fora conventional lambda sonde.

FIG. 4 shows how the signal from the sensor appears if it is installedin a system equivalent to that shown in FIG. 1. Sensor 10 is installedin the exhaust piping 7 g immediately downstream of the junction ofexhaust stubs 7 e and 7 f. The exhaust stubs 7 e and 7 f collect theexhaust gases from cylinders 2 a and 2 c, and 2 b and 2 d respectively.This type of exhaust gas system is used in four-cylinder internalcombustion engines where the order of ignition is 2 a-2 c-2 d-2 b, inwhich case the pressure pulse that is created in the exhaust gas valveopening should not affect the exhaust gas flow from the cylinder thathad opened its exhaust valve immediately beforehand. FIG. 1 shows arather asymmetrical exhaust gas system, but a symmetrical exhaust gassystem is to be preferred, in which every cylinder has the sameequivalent length of exhaust gas piping and union downstream to sensor10.

The four curves in FIG. 4 show the response of the sensor to a repeated(5 times) and identically rich combustion event in only one of the fourcylinders. The curves show, seen from the top, rich combustion incylinders 2 a, 2 c, 2 d and 2 b respectively, at an engine speed of 2400rpm. The response of the sensor to the rich combustion is shown as areduced voltage (SiC voltage). The upper curve in FIG. 1 shows thesignal from the sensor if the fuel supply to cylinder 2 a is beingregulated to achieve a λ value of about 0.92, while the λ values forcylinders 2 b, 2 c and 2 d are in the region of 1.0. The second curvefrom the top in FIG. 1 shows the signal from the sensor if the fuelsupply to cylinder 2 c is being regulated to achieve a λ value of about0.88, while the λ values for cylinders 2 a, 2 b and 2 d are 1.03, and1.0 respectively. In both these cases, the first and the second curvefrom the top, the overall excess air factor, i.e. as seen in thecombined exhaust gas flow from all the cylinders, is approximately 0.98.

FIG. 5a shows the excess air factors (λ) for cylinder 2 a (curve 1),cylinder 2 b (curve 2), cylinder 2 c (curve 3) and cylinder 2 d (curve4) as detected by a conventional lambda sonde inserted into eachindividual cylinder exhaust outlet, i.e. 7 a, 7 b, 7 c and 7 d in FIG.1, during the engine running period shown in the upper curve of FIG. 4.

FIG. 5b shows in an equivalent manner the excess air factor (λ) forthese cylinders during the engine running period shown in the secondcurve from the top in FIG. 4.

It can be seen from FIG. 4 how an individual rich combustion event caneasily be distinguished from surrounding lean combustion events. Theoutput signal from the sensor moves rapidly from a high to a low outputsignal level, which gives a typical binary signal characteristic. Thepulse width of the output signal from the sensor, or the length of timeit is in the lower signal level stage, differs from the expected quarterof the time period during the measurement, which is a consequence of thesensor's binary character, but also of the exhaust gas flow, the enginespeed profile and the diluting effect of the residual exhaust gases inthe exhaust piping. It can be seen from the upper curve in FIG. 4 thatthe sensor is indicating a lean air mixture of less than 1.0 for only18% of the time, instead of the nominal and expected 25% proportion ofthe time. For cylinder 3 a, the second curve from the top, which hasmuch richer combustion, see also FIG. 6, the pulse width shows that alean air mixture of less than 1.0 is indicated for approximately 40% ofthe total time. This phenomenon is utilised in the current invention inorder to be able to determine the relative richness in an individualcylinder, even if the sensor is installed in an arrangement where theflow of exhaust gas from several cylinders passes by in a specificorder.

With this specific sensor, information can thus be obtained on whethercombustion has taken place with too much or too little air, i.e. netoxidising or net reducing, for each individual combustion event, even ifonly one sensor is used in the exhaust pipe at position 7 g. At the sametime, the relative air deficit, here in the form of an excess of HC, canbe detected on the basis of the binary output signal pulse width.

If one also wishes to detect the relative deviation from stoichiometriccombustion from the air deficit side as well, i.e. for values exceeding1.0, a sensor combination can be employed using an oxygen-detectingsensor with equivalent characteristics.

With increasing richness a proportional increase of HC in the exhaustgases occurs, and with increasing leanness there is a proportionalincrease in oxygen. With selective binary sensors that are sensitive toHC and oxygen respectively, the relative deviation from the initialpoint, in either the direction of net reduction or net oxidation in theexhaust mixture, can be detected with the aid of the pulse widthinformation in the binary signals from the respective sensors. In thisway information obtained from two binary sensors can supply informationequivalent to that from a linear sensor, at a much lower cost.

In for example “Thin-film gas sensors based on semi-conducting metaloxides. Sensors & Actuators B23 (1995) 119-125. H. Meixner, J.Gerblinger, U. Lampe & M. Fleischer”, an oxygen-sensitive sensor withthe response that is required is described. This sensor combinationcould preferably be integrated on the same SiC substrate as the sensorshown in FIG. 3, thereby obtaining an integrated sensor matrix.

The actual pulse width of the binary signal can be determined by verysimple means. FIG. 1 shows how the signal from a sensor 10 of thisactual type is received by a comparator K, and as soon as the signalexceeds a reference voltage U the comparator provides a digital outputsignal to the engine control unit ECM. The engine control unit thenstarts a counter that determines the actual state of the signal when thedigital output signal from the comparator changes sign, i.e. the instantwhen the output signal from sensor 10 falls below the reference voltagelevel U. The presence of the digital output signal is equivalent to thepulse width from sensor 10, which is stored in the memory 11 of theengine control unit. The signal presence may either be related to aparticular time or to a number of crankshaft degrees through which theinternal combustion engine manages to rotate. Since the engine controlunit keeps track at all times of the crankshaft angle and engine speed,the pulse width can be matched to the cylinder that generated the richrunning signal. The mixture signal from sensor 10 always appears after acertain delay from the instant the exhaust gas valve from the respectivecylinder has begun to open.

If CD_(SIGN) defines the crankshaft position for the signal after theexhaust valve has begun to open at crankshaft position CD_(EO), thecrankshaft position for the signal is coarsely defined, since:CD_(SIGN)=CD_(EO)+f(rpm), where f(rpm) is a function dependent on theengine rotation speed.

f(RPM) is itself dependent on the actual geometry of the exhaust gascollection arrangement 7 a-7 g, and may, for a non-symmetric exhaust gascollector, be different for each cylinder.

The sequence of sensor signals from the exhaust gas pulses from thedifferent cylinders is identical to the ignition sequence. The enginecontrol unit can then use the measured pulse width to determine therelative richness and adaptively correlate the regulation so that thisis equivalent to the relative size of the richness deviation. After eachindicated richness signal the sensor pulse width information is kept inmemory as a value PW_(SIGN) _(—) _(CYL1) for example for cylinder number1, whereupon the engine control unit will initiate a reduction in theamount of fuel fed to cylinder 1 at the next fuel injection inlet event.The reduction of the amount of fuel injected can take place inpredetermined steps ΔT_(INJECT), where the next successive activationperiod for the injector T_(INJECT) _(—) _(NEXT) is provided by thefunction:

T_(INJECT) _(—) _(NEXT) _(—) _(CYL.1)=T_(INJECT) _(—) _(PREV) _(—)_(CYL.1)·(ΔT_(INJECT) * PW_(SIGN) _(—) _(CYL1)),

where T_(INJECT) _(—) _(PREV) _(—) _(CYL.1) is equivalent to theactivation period for the injector derived from the preceding richnessindication from the combustion event in cylinder number 1.

If the subsequent exhaust gas pulse from cylinder number 1 continues toindicate an over-rich mixture, a new value is obtained, PW_(SIGN+1). IfPW_(SIGN+1) for example happens to be 50% of PW_(SIGN), thepredetermined corrective step ΔT_(INJECT) can include a furthercorrection ΔT_(INJECT) _(—) _(Corr).

In this way the engine control unit can adaptively establish a matrix ofcorrection steps ΔT_(INJECT), where the actual correction stepΔT_(INJECT) is successively increased or reduced, by the factorΔT_(INJECT) _(—) _(Corr), if the regulatory measures do not returncombustion to a stoichiometric level within a certain successive numberof combustion events. The correction matrix is built up from at leastthe actual engine rotation speed and cylinder, whereby each individualcylinder can be corrected in an optimal way for every engine speedrange.

With the type of sensor being discussed, it is important that it isarranged to be as close as possible to the point where the exhaust gasesfrom several cylinders are combined. Optimally, the sensor should belocated only a few centimeters after the exhaust gas stubs join. Thefurther the sensor is located from the joining point, the more difficultit is for the sensor to distinguish individual over-rich combustionevents from neighbouring lean combustion events. For this reason, eventhe transport distances for the exhaust gases should be minimised, andthe whole exhaust gas collection system 7 a-7 f kept as compact aspossible.

The present invention can be utilised for at least the greater part ofthe internal combustion engine operating range. Detection cylinder bycylinder can be blocked during, for example, idling, where theregulation is mainly applied to obtain and maintain a stable enginerunning speed. During idling, i.e. at engine rotation speeds of lessthan 1,000 rpm, the exhaust gas flow pattern can be very irregular.

The present invention is not limited to the above-mentionedapplications. For example, a sensor can be arranged to be installed inthe exhaust gas collection systems for each bank of cylinders in a Veeengine. In other solutions a sensor may also be installed in the exhaustmanifold at a point where the exhaust gases from only two cylinders arecombined. The important thing is that the relative richness of anindividual cylinder can be detected in the combined gas flow fromseveral cylinders. One may also use a combination of the sensor underdiscussion with a conventional lambda sonde. The conventional lambdasonde can supervise the combined gas flow and retain the detected valuefor maintaining an exhaust gas blend that is optimal for a catalyticconverter. If, for example, the lambda sonde indicates that the totalexhaust gas flow has a correct blend, an individually over-rich fuel-airmixture in one cylinder mean a reduction in the amount of fuel deliveredduring the next inlet event to that cylinder, while the other cylinderswill receive a leaner fuel-air mixture. The leaner combustion in theother cylinders can however be limited or reversed if these afterenrichment indicate over-richness from the binary sensor at their nextcombustion events. The sensor under discussion can best of all becomplemented by a conventional lambda sonde with transients, i.e. onapplying load, where more fuel is to be ramped, depending on the desiredincrease in engine power output. A problem connected with this is thatit is more difficult to rapidly increase the air mass, so that fuel maybe over-dosed at the initial stage of increasing load. Any over-richnessduring load application is detected immediately after every combustionevent, and if a limited amount of extra rich injection shall bepermitted, so one may during regulation permit additional fuel to besupplied sequentially to the different cylinders.

What is claimed is:
 1. A regulating system for regulating an air-fuelmixture in an internal combustion engine including a plurality ofcylinders, connected to an exhaust gas system including at least onepoint where exhaust gases from at least two of the cylinders are joined,the regulating system comprising: a first binary sensor arranged forsensing the exhaust gases and producing a first output signal having aswitching point from a first output signal level to a second outputsignal level, where the first output signal level is stable for so longas the air-fuel mixture is the exhaust gases is lean, and where thesecond output signal level is achieved when the air-fuel mixture in theexhaust gases is rich, the first binary sensor being located at the atleast one point; and an engine control unit comprising first means forregulating the amount of fuel delivered to each of the plurality ofcylinders in the engine depending upon at least one actual engineparameter, and applying a correction to the amount of delivered fueldepending on the first output signal from the first binary sensor;second means for matching from which of the plurality of cylinders thesensed exhaust gases derive, third means for detecting a pulse width ofthe second output signal level of the first output signal and storingthe pulse width as a first value in a memory as a combustion-relatedvalue; fourth means arranged for ascertaining a level of richness of theair-fuel mixture in the exhaust gases in relation to the first value aswell as actual operating conditions of the engine, including enginerotation speed provided by an engine rotation speed sensor; and fifthmeans for reducing the amount of delivered fuel to only that of theplurality of cylinders which after matching is indicated as having arich mixture in the exhaust gases.
 2. The regulating system of claim 1,further comprising a second binary senor arranged for sensing theexhaust part and producing a second output signal having a switchingpoint from a first output signal level to a second output signal level,where the first output signal level is stable for so long as theair-fuel mixture in the exhaust gases is not lean, and where the secondoutput signal level is achieved when the air-fuel mixture in theexhausted gases is lean, the second binary sensor being located at theat least one point.
 3. The regulating system of claim 2, wherein theengine control unit further comprises sixth means for detecting a pulsewidth of the second output signal level of the second output signal andstoring the pulse width of the second output signal level of the secondoutput signal as a second value in the memory as anothercombustion-related value; seventh means arranged for ascertaining alevel of leanness of the air-fuel mixture in the exhaust gases inrelation to the second value as well as actual operating condition ofthe engine, including engine rotation speed provided by the enginerotation speed sensor; and eighth means for increasing the amount ofdelivered fuel to only that of the plurality of cylinders which aftermatching is indicated as having a lean mixture in the exhaust gases. 4.The regulating system of claim 2, wherein the first binary sensor andthe second binary sensor are arranged on one semi-conducting substrate.5. The regulating system of claim 4 wherein the semi-conductingsubstrate comprises Silicon Carbide.
 6. A process for determining anair-fuel mixture in a plurality of cylinders in an internal combustionengine including a binary sensor arranged in an exhaust gas systemimmediately downstream of at least one point where exhaust gases from atleast two of the plurality of cylinders are joined, the processcomprising the steps of: providing an output signal from the binarysensor, the output signal having a switching point from a first outputsignal level to a second output signal level, where the first outputsignal level is stable for so long as the air-fuel mixture in theexhaust gases is not rich, and where the second output signal level isachieved when the air-fuel mixture in the exhaust gases is rich;detecting angular engine position, thereby determining which of the atleast two cylinders emitted the exhaust gasses sensed by the binarysensor, wherein each of the at least two cylinders can be matched to theoutput signal in a sequential order equivalent to an ignition sequencein the at least two cylinders, detecting a pulse width of at least oneof the output signal levels, which pulse width can be measured in termsof time or crankshaft angle; and determining a relative deviation fromstoichiometric combustion in the matching cylinder based upon thedetected pulse width.
 7. The process of claim 6, wherein the secondoutput signal level indicating the air-fuel mixture in the exhaust gasesas being rich, is used for further determining which of the at least twocylinders received excess fuel.